1. Field of the Invention
The present invention relates to filling machines of the rotary peristaltic pump type and, more particularly, to an apparatus and method for improving the dispensing accuracy of such peristaltic pump filling machines.
The use of electronically controlled filling machines or pumps that dispense gases, liquids, semi-liquids, pastes, powders and solids prior to packaging is well known in the prior art and is becoming widespread in many industries. These machines typically include a product supply means, a driving device to propel the supplied product and filling nozzles for directing the product into a collection container Assisting devices such as shut-off valves at the output of the nozzle or a so-called suck-back system for dripless and clean operation are sometimes used to assure proper and clean operation of the filling machine.
Filling machines are generally classified by the type of driving device used (e.g. gravity type for dispensing solids and liquids, piston type for dispensing liquids and gases, rotary pumps for dispensing liquids and gases, and auger screw and vibrators for dispensing solids and powders).
In gravity fillers the product is driven by gravity through a controlled valve into the nozzle and collection container. The flow rate of the product in gravity type fillers is relatively uniform, and the amount of product dispensed is controlled by adjusting the time for closing the valve. In more precise systems weight feedback is used to control the volume of dispensed product.
In piston driven filling machines, the product enters the dispensing cylinder by opening an infeed valve, moving the piston in a reverse direction, closing the infeed valve, opening a discharge valve, and driving the piston in the opposite direction so that the product is propelled to the nozzle and into the collection container. The volume of the filled product is controlled by adjusting the stroke of the piston.
In rotary pumps and augers, the volume of product is controlled by timing the actuation and stopping of the driver, by controlling the angular distance of the driver or by stopping the driver upon receiving feedback on the amount already dispensed.
The filling machines described above generally treat the flow rate or alternatively the dispensed volume as a periodic function of driver position. For example, in the case of gravity filling, each cycle uses the same parameter values for activating and stopping a driver. In the case of piston filling, each cycle uses the same stroke to deliver a given fill volume. In the case of an auger screw or rotary pump, each angular rotation is treated as delivering the same volume.
The present invention relates to a rotary peristaltic pumps which are typically regarded as delivering a fixed volume for each angular rotation of the driver. The flow rate of the peristaltic pump has been compared to the flow of product pumped by a continuously running piston filler. In the case of the piston filler, in order to achieve fill precision in each collection container, it may be necessary to control the piston movement, so it will move an integer number (including zero) of piston strokes plus a fraction of the next stroke depending on the fill weight required and the volume of the filling cylinder. In conventional rotary peristaltic pumps the quantity of product delivered is also regarded as a fixed volume per revolution of a dispenser pump.
Such a rotary peristaltic pump may be rotated by a variable-speed electric motor, a servo-motor, a stepping motor, or other suitable drive mechanism
Rotary peristaltic pumps are preferred for many filling applications due to their ability to pump fluids through tubing without any contact between pump components and the fluid being pumped. In a typical rotary peristaltic pump system, one or more lengths of tubing are contacted by a series of rollers that generally rotate in a circular path so as to squeeze the tubing against a curved wall surface. This provides a moving region or regions of compression along the length of tubing. Movement of the compressed region of the tubing forces fluid ahead of the moving region, and the action of the tubing in returning to its uncompressed condition creates a partial vacuum, which effects forward flow of the fluid from the region behind the compressed region Rotary peristaltic pumps operate such that the material first introduced is the material first expelled to avoid the possibility that stagnant pockets of product may occur which result in breeding places for potentially harmful bacteria, thus posing sanitary problems.
It has been heretofore assumed that repeating cycles of the same angular rotation of a rotary peristaltic dispenser will deliver consistent quantities of product. However, a problem typically associated with such rotary peristaltic pumps is that it is difficult to obtain accurate and repeatable volume dispensing from them.
2. Description of Related Prior Art
Several prior art patents have dealt with improvements in pumping accuracy. One example of such a prior art device is disclosed in U.S. Pat. No. 4,715,786 (""786 patent) which enhances accuracy by direct measurement of parameters relating to volume dispensed, motor speed and flow rate. These readings are accumulated in counters that are updated on a real time basis. The contents of the counters are used to increment other counters and can be reset at particular real time intervals. By accumulating exact counts relating to volume, motor speed and time, the parameters of flow rate, revolutions per minute and cumulative volume are calculated. One aspect of the (""786) patent includes the calculation of a calibration constant relating known increments of angular rotation of the pump to a known fluid volume. This constant is used to determine flow rate and total or cumulative volume by counting angular increments of pump rotation, and assumes that for every angular displacement of the pump head, a known constant quantity of fluid is pumped.
Another prior art method for obtaining enhanced accuracy in peristaltic pumps is disclosed in U.S. Pat. No. 4,910,682 wherein the calibration constant is compared against a predetermined value, which causes the control to enter into either a high flow mode or a low flow mode. In the low flow mode, volumes are resolved to a higher accuracy than in the high flow mode. However, under this method almost the entire quantity of product is dispensed in the high flow mode. Thereafter, an intermittent operation of the pump brings the quantity dispensed to the desired volume. This invention again assumes that for every angular displacement of the pump head, a known constant quantity of fluid is pumped.
Encoder wheels have also been used in the prior art to improve the accuracy of rotary peristaltic pumps by monitoring the rotation of the drive shaft in small angular sectors. An example of the use of such an encoder wheel is provided in U.S. Pat. No. 5,003,239.
U.S. Pat. No. 5,733,257 discloses a method for calibrating a peristaltic pump comprising an internal fluid flow meter. Fluid is introduced into the pump segment and is pumped by the peristaltic pump at a substantially constant rotation rate. Three different inlet pressures are obtained and measured by a pressure meter and the corresponding fluid flow is measured by the internal fluid flow meter for obtaining calibration pair values. A calibration curve is calculated from said pair values by a computer inside the filling machine. The actual fluid flow rate is determined by the computer from the calibration curve based on the actual inlet pressure and the actual revolution rate of the propelling means.
In each of the prior art examples recited hereinabove, it is assumed that the same angular distance of the driver will cause the delivery of the same volume of the product once the pump has been calibrated. However, this simple calibration factor or calibration coefficient takes into account only the physical characteristics of the pump and associated tubing. This model assumes that the relation between the distance of the pump rotor and the dispensed volume of the product is a linear function with a constant coefficient linking two variables: volume and angular distance. The problem with this model is that the real flow of a rotary peristaltic pump has a pulsating character wherein the number of pulses in a single 360 degree revolution of the driver or rotor is equal to the number of rollers of the pump.
When the simple calibration factor described above is utilized, a relatively large absolute error in the quantity of dispensed product results. This error is larger when peristaltic tubes of a larger inner diameter are used to achieve high production speeds. The error is even more significant when filling small volumes.
In order to reduce the absolute error created by the simple calibration factor, more rollers can be utilized to dispense the product. This results in a higher frequency of pulses and a lower degree of amplitude.
Recognizing that the pulsating character of peristaltic pumps affects flow, some prior art pumps have attempted to reduce flow pulsation. One prior art example for reducing pulsation in peristaltic pump outflow is set forth in U.S. Pat. No. 4,834,630 which discloses a segmented rotor having rollers in a first segment being staggered or alternated with respect to rollers in a second segment, with each segment engaging a plurality of fluid conduits, and with each tube engaged by the first segment being connected by a T-shaped coupler to one engaged by the second segment on the output side of the pump. Another technique that has been used is to employ twin tubes engaged by a pair of offset, spring-loaded tracks in a single peristaltic pump-head, with the flow form the twin tubes directed to a single tube by a Y-connector.
Still another prior art example limiting the effect of flow pulsation is disclosed in U.S. Pat. No. 5,257,917 wherein a peristaltic pump comprises a rotor and a plurality of removable cartridges associated with the rotor, wherein occlusion beds of the cartridges are configured to enable the outflow characteristics of the pump to be varied by manipulation or interchanging of the cartridges, such that the pump may in one operation have synchronous flow to all of its parallel flow channels, or may in a second mode of operation have non-synchronous phase-offset flow to respective ones of the parallel flow channels. In the second mode of operation, manifolding of the output flow from respective ones of the parallel flow channels can be employed to provide flow of substantially reduced pulsation.
The weakness of these models is again the assumption that the relation between the distance of the pump rotor and the dispensed volume of the product is a linear function with a constant coefficient linking the volume and angular distance of the driver. Although reduction of pulsation reduces the error arising from the assumption of a linear relationship between volume and angular distance of the driver, there is much room for improvement by not treating the relationship between flow rate or volume and the angular distance of the driver as linear.
Accordingly, the present invention provides a filling machine with a rotary peristaltic dispenser that compensates for the nonlinear, real flow rate characteristic of the dispenser so as to increase product fill accuracy. In the present invention a computer processor with associated electronic circuitry and software programs are provided for defining, digitizing and storing the real pulsating flow rate characteristic of the rotary peristaltic dispenser. In order to ensure a precise delivery of the filled product, the computer calculates the angular rotation of the rotor needed to deliver the desired fill weight for each filling cycle based on the nonlinear, real flow characteristic of the dispenser pump. Because the angular rotation needed to maintain a uniform fill weight is different for each filling cycle, the computer processor continuously monitors the position of the dispense rotor in relation to the flow characteristic. The current position of the rotor is established prior to each filling cycle either by calculation from a signal provided by a sensing means such as a position transducer to sense the position of the rotor, or by a signal sent directly to the driver. The current position is then located on a look-up table stored in the computer containing the flow characteristic as a starting point for digital integration of the characteristic over the distance of the driver.
The computer then integrates the digitized flow characteristic based on the look-up table starting from the current rotor position one step at a time. Each step represents a small regular angular increment of the dispense rotor. After each step of integration, the subtotal of the calculated amount is compared to a calibrated set-point until the desired set-point fill weight is reached. When the integrated subtotal reaches the set-point, the integration stops. The number of steps used during the integration establishes the target distance of the system driver. The present system then advances the driver by the calculated angular distance thereby ensuring the delivery of the required product weight with high precision.
Thus, the apparatus and control system of the present invention take into account the pulsating character of the peristaltic dispenser. This is fundamentally different from prior art methods, in which the filling stroke of the rotor is simplified to be a linear constant. While the present invention is illustrated for the filling of liquids, it is equally applicable for filling other products such as gases, semiliquids, pastes, viscous and nonvisous and solid products, either by weight or volumetric amounts where the flow characteristic is a periodic function of a system driver position.
Other features and advantages of the present invention will become apparent from a study of the following description and accompanying drawings.